Tree harvesting machines such as feller buncher machines have saws for cutting off trees. It is typical for such saws to be driven by hydraulic motors. Many tree harvesting machines have circular saw blades for cutting off trees and/or bucking logs to length. Such circular saw blades typically comprise a rotatable disc with cutting teeth mounted around the periphery of the disc. The saw may be called a “cutting element”.
In addition to the cutting element, some tree harvesting machines have hydraulic actuators, which control the advance of the cutting element into a tree or log. These actuators may be called a “feed mechanism”.
When harvesting trees or when bucking logs to length, the torque required to drive the cutting element varies depending on many factors including, inter alia, tree diameter, wood density, location of the cutting element within the tree, localized characteristics of the tree (i.e. knots) and speed of the feed mechanism. Typically, the torque required to cut through a relatively large tree will be greater than that required to cut through a smaller tree. Also, the torque required to drive the cutting element will typically increase as the cutting element moves from the outside of a tree to its center and then will decrease again as the cutting element moves from the center of the tree to its opposite side.
Typically, tree harvesting machines are built with hydraulic motors that rotate the cutting element to cut through trees and logs. In order to successfully cut through large trees and logs, the hydraulic motors must be configured to provide significant torque. However, high torque hydraulic motors rotate the cutting element relatively slowly. The result is a lack of cutting efficiency for smaller trees and logs, which may be cut faster with motors making more revolutions per minute.
To solve this problem, some wood cutting machines employ two hydraulic motors, one configured to operate at high torque (but at low speed) and the other configured to operate more quickly (but at lower torque). This solution is inadequate, because of the cost of providing, operating and maintaining hydraulic motors and the time involved to change from one motor to the other in different operating conditions.
Manual-advance tree harvesting machines may also be equipped with high-speed (or variable speed) motors. Manual-advance tree harvesting machines may not be effective for larger trees and rely on the skill of the operator to feed the cutting element through the tree at an appropriate rate. In addition, manual advance tree harvesting machines may subject their operators to a greater risk of injury.
Another known technique involves the use of a two-speed feed mechanism. The rate of advance of the cutting element into the tree can be reduced when the motor driving the cutting element is experiencing high torque. This technique does not provide a mechanism for controlling the speed of the cutting element itself.
There is a need for hydraulic systems for saws that improve tree and/or log cutting efficiency and ameliorate some of the drawbacks of the prior art.